How times have changed. Karp announced at Ad Age's Digital
Conference that on May 2, Tumblr will start offering advertisers
the ability to buy an ad unit on Tumblr Radar, which highlights
the site's top posts and gets approximately 120,000,000
impressions a day.

"I was probably being an idiot then," Karp said of his earlier
renunciation of all advertising. His conversion is relatively
recent—on April 12 he told Ad Age that advertising was "a
complete last resort."

Tumblr spokesperson Katharine Barna added that the real estate
being offered to advertisers was "not an 'ad unit' per
se, but ... outside of the blogs you follow [on your
dashboard] there are places that we highlight certain posts and
blogs. We're incredibly excited to finally open up some of
this real estate to the most creative brands on Tumblr."